Method of and joint for electrofusion coupling of thermoplastic pipes

ABSTRACT

A method and apparatus are disclosed for a thermoplastic pipe joint. The pipe joint consists of a channel machined or routed into each end of the pipes to be joined and a coupler manufactured from substantially the same type of thermoplastic material as the pipes, with an electrical resistance element embedded into the coupler. Application of power to the electrical resistance element raises the temperature of the interface between the coupler the pipe walls causing the thermoplastic material to form a fusion bond between the coupler and the pipes, resulting in a leak free joint with no interior flow obstructions or exterior collars or protrusions. There is also disclosed a method for electrofusion welding solid wall thermoplastic pipes without leaving an interior or exterior obstruction by machining a socket and nipple on the pipes to be joined and adding an electrical resistance element to the nipple.

This is a divisional of application Ser. No. 08/654,104 filed May 28,1996, which is still pending.

FIELD OF THE INVENTION

This invention relates generally to the electrofusion welding of pipesmade from thermoplastic materials including polyethylene, polyvinylchloride, nylons, polybutylene, polypropylene and the like.Specifically, this invention relates to electrofusion welding of suchpipes to result in a joint which has substantially flush interior andexterior surfaces and provides constant inside and outside diameters.

BACKGROUND OF THE INVENTION

This invention pertains to the joining of thermoplastic pipes includingpolyolefin, polyethylene, polyvinyl chloride, nylon, polybutylene,polypropylene and the like. These types of materials are gainingpopularity in water, sewer, culverts and industrial piping because oftheir characteristics of being lightweight, corrosion resistance, strongand durable. "Trenchless" rehabilitation of culverts, storm sewers,sanitary sewers and other underground pipes, by "slip lining" or "insertrenewal" with thermoplastic pipes is gaining popularity and growingrapidly throughout the United States and other countries. In thisprocess, a thermoplastic pipe or liner is inserted into an existing pipeor culvert without removal of the deteriorated pipe. The replacementpipe is pushed into or pulled through the existing culvert. In manycases, an existing pipeline can be rehabilitated for a fraction of thecost of replacement and with minimal inconvenience to the public.

Thermoplastic pipes, including polyethylene, are the preferred pipematerial for many rehabilitation projects because of the price and theabove-noted characteristics. Generally, thermoplastic pipe ismanufactured in lengths which are sufficiently short to permittransportation and handling. In the field, where the pipe is to beinstalled, the short pipe sections must be connected to form acontinuous pipe of a predetermined length appropriate for theapplication.

The joining or connecting of thermoplastic pipes can present manyproblems because of the variety of field conditions encountered andbecause of thermoplastic pipes chemical resistance, which, in manycases, makes such pipes impervious to glues or cements. Also, somethermoplastic pipes have a tendency to "creep," or move, when subjectedto changing temperatures. Because most applications include exposure tosuch temperature changes, such movement or "creeping" limits the abilityto use mechanical type joints such as threads.

In general, several methods exist to join thermoplastic pipe in thefield. A first method is "butt fusion" as it is known in the art. Thismethod involves the use of a butt fusion machine which includes line upequipment and a hot plate. The ends of two pipes to be joined areinserted into the line up equipment which aligns and advances the pipeends toward one another as necessary. The two pipe ends are pressedagainst a heat plate which heats and softens the two pipe ends. The heatplate is then removed and the line up equipment advances the two pipestoward one another at a predetermined rate in order to fuse the pipeends together. This type of butt fusion requires special fusionequipment that is expensive and not always available in the field.

Another method is the use of electrofusion collars or inserts. One typeof electrofusion collar is shown in U.S. Pat. No. 4,530,521 to Nyffeler,et al. One type of electrofusion insert is shown in U.S. Pat. No.3,768,841 to Byrne at al. These devices, as shown in the references, usea sleeve, collar or insert made of thermoplastic material and whicheither fits over or into is the pipes to be joined. The pipes, and thecollar or insert are first heated to soften the thermoplastic material.If using a collar, the pipe ends are inserted into the collar and arethereby joined. If using an insert, the insert is inserted into eachpipe end thereby joining the pipes. The heating can be performed withfusion equipment or the collar or insert can contain an electricalelement to allow electrofusion welding of the pipes to the collar orinsert.

These devices have the disadvantage of interior obstructions or exteriorprotrusions which are not acceptable in many applications. Because thecollar must be large enough to accept insertion of the pipe ends, theresulting joint does not have a flush exterior. Because the insertreduces the inside diameter of the pipes at the joint, the insert actsas an obstruction to flow through the pipe. This is unacceptable in mostapplications, including most trenchless rehabilitation projects, becauseinterior flow obstruction is not acceptable. Furthermore, exteriorcollars impede insertion of the replacement of the pipe during sliplining and require the use of smaller diameter replacement pipes so thatthe collar can fit over the pipe and inside the existing pipe orculvert.

Another method of joining thermoplastic pipes is electrofusion rods ormesh as shown in U.S. Pat. No. 5,410,131 to Brunet et al. Although thisdevice requires no collar or insert, the application requiressubstantial end pressure to join the two pipe ends. Such pressure isusually supplied by special line up equipment due to the weight of thepipes and this equipment is expensive and not always available orpractical for use in field conditions.

Another method, hot air gun welding uses a welding rod of thermoplasticmaterial fed through the nozzle of a hot air gun. The hot air gunapplies heat to the ends of the pipe to be welded and melts the weldingrod which is applied to a bevel cut between the two ends of the pipe tobe joined. Line up equipment must be used, as with the butt fusionmethods, and this method has not proven satisfactory in field conditionsdue to a lack of uniformity in the welding process.

As thermoplastic resin prices increase, plastic pipe manufacturers areconstantly looking for manufacturing methods to make pipe lighterwithout losing physical strength. One type of thermoplastic pipe gainingpopularity is "profile wall pipe" as it is known in the art. An exampleof this type of pipe is shown in U.S. Pat. No. 5,362,114 to Levingston.Profile wall pipe is thermoplastic pipe formed by extrusion to have aninner cylindrical wall, a generally concentric outer cylindrical walland a helical rib between and connecting the inner wall and outer wall.

Profile wall pipe is lighter than solid pipe and uses less materialthereby reducing resin costs, but it maintains a high degree ofstrength. Because it is lighter than solid wall it generally has acompetitive advantage over solid wall plastic pipe. Profile wall pipe ispopular in the industry because it uses less material and is lighter inweight than solid thermoplastic pipe. Unfortunately, typical methods offusing thermoplastic pipe are unacceptable and do not work on profilewall pipe because the end walls of such pipe do not present a solidannular surface due to the presence of the helical rib contained in thepipe.

For example, butt fusion is very difficult on profile wall pipe becausethe pipe ends of profile wall pipe are not solid. The profile wall pipeends have a thin inner wall, a thin outer wall and a "profile space" thedepth of which is equal to the distance between the end of the pipe andthe helical rib which connects the inner and outer walls. The sameproblems that exist in joining solid wall thermoplastic pipes aremultiplied in profile wall pipes because of their relatively thin innerand outer walls with a large profile space between the walls.

For example one manufacturer produces a polyethylene 10" inch insidediameter profile wall pipe that has inner and outer wall thicknesses of0.065" inches to 0.079" inches. This particular pipe has a profiledistance over 1 inch between the inner and outer walls with an outsidediameter of approximately 11.20" inches. A 36" inch inside diameterprofile wall pipe from the same manufacturer has inner and outer wallthicknesses of approximately 0.195" to 0.228" inches and an outsidediameter of approximately 40.65" inches giving a profile distancebetween the inner and outer walls over 4 inches.

Butt fusion of profile wall pipes is very difficult due to the thin wallthickness compared to the overall diameter. Setting the correcthydraulic pressure on a butt fusion machine for such thin walls andlarge diameters would result in extremely slow fusion machine carriagemovement and potential cooling of the thermoplastic prior to fusionjoining. This results in a failed weld or "cold joint." Rods or meshalone also will not work on profile wall pipe because profile wall pipedoes not have solid flat pipe end surfaces which are required in thosemethods.

Hot gas welding on profile wall pipes will not achieve a strong, uniformjoint even with the most experienced welders. Electrofusion collars orinserts would be very bulky and either seriously interrupt interiorflows or have a very large collar on the outside of the pipe, making thepipe joint unsuitable for slip lining or pipe rehabilitationapplications.

One method for joining profile wall pipes is shown in U.S. Pat 5,362,114to Levingston. As shown therein, profile wall pipe is joined by threadedengagement. The threads, formed by the helical rib, are revealed byshaving away the inner wall of one pipe section and the outer wall ofanother pipe section. This allows the two pipe sections to be threadedtogether. However, this method requires the use of sealants or gasketsto make the joint water tight or leak-proof and does not provide thebeneficial characteristics of a fusion welded joint. For example, afusion weld is also air tight whereas a threaded joint is not acceptablefor gas pipelines which require air tight seals.

Thus, prior art collars or inserts result in interior flow obstructionsand or collars or exterior protrusions which would prevent the pipeinsertion in trenchless applications. Prior art welding rods or meshrequire line up equipment to push the ends of the pipes together insolid wall pipe and would not be acceptable for the joining of profilewall pipes. Prior art threaded methods for joining profile wall pipesare not leak proof without sealants or gaskets. Furthermore,specifications in many applications call for a leak-proof joint that hasa flush interior and exterior pipe surface and in trenchlessapplications is strong enough to withstand pulling or pushing the pipethrough an existing pipeline. The above type methods do not satisfythese specifications.

SUMMARY OF THE INVENTION

The present invention results in a uniform, strong, leak-proof jointwith no inner or outer obstructions, making it suitable for trenchless,slip line applications, in addition to direct burial applications. Thepresent invention requires no fusion machine, or special line-upequipment to apply end pressure. The present invention also uses anelectrical resistance screen element which is an improvement overresistance wires in both cost and the uniform heat distribution a screenprovides.

In the present invention, profile wall or solid wall pipe is furnishedwith a channel machined or routed in an end wall between the inner andouter walls of the pipe. This channel may also be field fabricated withthe use of a routing tool mounted on a plate with roller guides, toprevent it from digging into the inner or outer walls of the pipe, andprovides an even groove to a predetermined depth. An electrofusioncoupler manufactured from like thermoplastic material with a stainlesssteel electrical resistance element embedded into a surface of thecoupler is inserted into the channel. A terminal pin is attached to eachend of the electrical resistance element for connection to a powersource.

The coupler is configured to provide a tight interference fit betweenthe inner and outer walls of the profile wall pipe. The ends of thecoupler are slightly beveled or chamfered to facilitate the insertion ofthe coupler into the channel of each pipe. To form the pipe joint, thecoupler is pushed into the channel in the adjacent end walls of thepipes to be joined. A power source is connected to the terminal pins ofthe electrical resistance element which protrude from the joint.

Preferably an electrofusion control unit furnishes a predeterminedelectrical current required to heat the electrical resistance elementand partially melt the thermoplastic coupler and the pipe walls. Thecoupler during the heating cycle expands at a greater rate than the pipewalls due to the attachment of the electrical resistance elementthereon. This develops additional internal bonding pressures to producea fusion bond of adequate depth and continuity to form a pressure tightpipe joint, that is substantially flush with the pipe walls, leaving nointerior flow restrictions or exterior collars or obstructions.

In the case of a solid wall thermoplastic pipe, the present invention isprovided on the exterior wall of one pipe end that has been machined orshaved approximately one half of the wall thickness on the exteriorportion of the pipe wall thereby forming a nipple. On the second pipe tobe joined, the interior wall of the pipe is machined or shavedapproximately one half of the wall thickness, thus forming a socket.Upon insertion of the nipple into the socket, a pipe joint is formedwithout any collar or other exterior or interior obstructions.

An electrofusion coupling is formed by embedding a stainless steelelectrical resistance element into the exterior surface of the nipple. Aterminal pin is attached to each end of the electrical resistanceelement to allow connection to a power source outside the joint. Thenipple and socket are machined to provide a tight interference fitbetween the two pipes. The edge of the nipple is slightly beveled orchamfered to facilitate the insertion of the nipple into the socket.

To form the pipe joint, the nipple containing the electrical resistanceelement is inserted into the socket. A power source is connected to theterminal pins, preferably an electrofusion control unit furnishing apredetermined electrical current required to heat the mesh or resistancewires and partially melt the nipple and socket thereby forming a fusionweld. The nipple expands at a greater rate than the socket due to theattachment of the electrofusion element thereto. This developsadditional internal bonding pressures to produce a fusion bond ofadequate depth and continuity to form a pressure tight pipe joint thatis substantially flush with the pipe walls and leaves no interiorobstructions or exterior collar.

Also provided is the use of a single continuous piece of stainless steelor other metal screen as the electrical resistance element. Theadvantage of a one piece screen versus a helical heating coil or mesh isa complete coverage of the required heating area without the potentialto short out due to movement of the wire or loss of insulation on themesh. This method in production may be considerably less expensive thatother resistance heating methods.

While the principal advantages and features of the invention have beendescribed above, a greater understanding of the invention may beattained by referring to the drawings and the description of thepreferred embodiment which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of two square profile wall pipesshowing the coupler between the two pipes prior to being joined.

FIG. 2 is a cross sectional view of the two square profile wall pipesand the coupler therebetween prior to being joined.

FIG. 3 is a perspective view of the coupler with the electricalresistance element placed thereon.

FIG. 4 is a side view of the coupler of FIG. 3.

FIG. 5 is a cross sectional view of two square profile wall pipesunited, end wall to end wall, with the coupler in the channel of eachpipe.

FIG. 6 shows a flat electrical resistance element screen prior toattachment to the coupler.

FIG. 7 is a perspective view of a terminal pin before insertion into thescreen.

FIG. 8 is a direct view of a coupler with an electrical resistanceelement in the form of a helical wire wound around coupler.

FIG. 9 is cross sectional view of two round profile wall pipes with thecoupler between the pipe sections after being joined and electrofusionwelded.

FIG. 10 is cross sectional view of two square profile wall pipes showingthe coupler therebetween prior to being joined.

FIG. 11 is cross sectional view of two round profile wall pipes showingthe coupler between the pipe sections and in the channel of each pipe.

FIG. 12 is a cross sectional view of two round profile wall pipesections after being joined and electrofusion welded.

FIG. 13 is a dimensional view of two solid wall thermoplastic pipesadjacent to each other with the inside wall of one pipe having a nippleand one pipe having a socket.

FIG. 14 is a direct view of two solid wall thermoplastic pipes adjacentto each other with the inside wall of one pipe having a nipple and onepipe having a socket.

FIG. 15 is a perspective view of two solid wall thermoplastic pipesadjacent to each other with the inside wall of one pipe having a nippleand one pipe having a socket.

FIG. 16 is a cross sectional view of the two solid wall thermoplasticpipe sections of FIG. 15 after insertion of the nipple into the socket.

FIG. 17 is a cross sectional view of the two solid wall thermoplasticpipe sections of FIG. 16 after electrofusion welding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A first embodiment of the present invention is shown in FIG. 1.Identical profile wall pipes 10 are shown. Because the pipes areidentical in this embodiment of the invention, only one pipe will bedescribed in detail, however, it is understood that the detaileddescription of the pipe in this embodiment applies equally to each pipeunless otherwise specified. Profile wall pipes 10 comprise a generallycylindrical outer wall 12, a cylindrical inner wall 14 substantiallyconcentric with outer wall 12 and a helical rib 16 located generallybetween the walls 12 and 14 and joining the walls 12 and 14 together.The convolutions of the rib 16 extend in multiple turns around theexterior of the inner wall 14 and along the interior of the outer wall12 for substantially the entire length of each pipe 10. Rib 16 advancesincrementally along the longitudinal axis of pipe 10 as it winds aroundpipe 10 and thereby forms a profile space 11 between each wind of therib 16 around the pipe 10. Both outer wall 12 and inner wall 14terminate at an end wall 18.

In the preferred embodiment, the profile wall pipe is polyethylene pipe10 manufactured by extrusion so that each length of pipe is one integralpiece of material. Outer wall 12, inner wall 14 and rib 16 all extendthe full length of pipe 10. Outer wall 12 and inner wall 14 haverelatively smooth surfaces and terminate at end wall 18. Rib 16 is notflush with the plane of the end wall 18 for the entire circumference ofpipe 10 at end wall 18 because the helical rib 16 advances incrementallyalong the longitudinal axis of pipe 10 with each turn around pipe 10.Each turn of rib 16 represents 360 degrees of angular extension of rib16 about the longitudinal axis of pipe 10.

Profile space 11 is formed during the manufacture of profile wall pipe10 and bordered on the exterior by outer wall 12, on the interior byinner wall 14 and on each side by rib 16. Profile space 11 can besquare, as shown in FIG. 2 or round, as shown in FIG. 10, depending onthe extrusion process. Either round or square profile pipe is acceptablefor the present invention.

Referring to FIG. 2, rib 16 advances along the longitudinal axis of pipe10 as it traverses the length of pipe 10 and connects outer wall 12 andinner wall 14. In profile wall pipes, before application of the presentinvention, the depth of profile space at an end of pipe 10, measuredfrom the plane of end wall 18, increases along the circumference of endwall 18. The amount of increase in the depth of the profile space is afunction of the longitudinal advance of rib 16 with each turn aroundpipe 10.

In the preferred embodiment, shown in FIG. 2, endwall 18 of pipe 10 hasa channel 20 formed therein for receiving coupler 22. Channel 20 isformed by removing a portion of rib 16 between outer wall 12 and innerwall 14. The amount of rib 16 which is removed depends on the desireddepth of channel 20. The depth of channel 20 must be equal to or greaterthan a predetermined value which is dependent on the size of coupler 22as described below. In the preferred embodiment, the depth of thechannel 20 must equal or exceed the predetermined depth for the entirecircumference of end wall 18.

Coupler 22, shown in FIG. 3, is an annular ring having an exteriorsurface 24 and an interior surface 26. Coupler 22 is configured with aninside diameter and an outside diameter to allow insertion of coupler 22into channel 20. However, the configuration of coupler 22 must alsoallow mating of coupler exterior surface 24 with outer wall 12 of eachpipe 10 and mating of coupler interior surface 26 with inner wall 14 ofeach pipe 10.

Referring to FIG. 4, coupler 22 has an axial width, measured from edge28 to edge 30, which is determined by the size of pipe to be joined. Thedepth of channel 20 (FIG. 2) in the preferred embodiment must be equalor greater than 1/2 the width of coupler 22. Coupler 22 has a thickness,measured from exterior surface 24 to interior surface 26, which isapproximately equal to the distance between inner wall 14 and outer wall12 of each pipe 10. This allows for an interference fit of coupler 22within channel 20 of each pipe. The edges 28 and 30 of coupler 22 arebeveled or chamfered to facilitate insertion into channel 20.

Referring to FIG. 5, upon insertion of coupler 22 into channel 20 ofeach pipe 10, the end walls 18 of the pipes 10 should abut against oneanother to allow for a substantial flush pipe joint with a substantiallycontinuous and flush inner wall and a substantially flush and continuousouter wall. As set forth above, a pipe joint with substantially flushinner and outer walls provides for a pipe joint which will not impedeflow through the pipe nor impede insertion of the joined pipes in anexisting culvert. The present invention also allows the entire length ofjoined pipe to have a substantially uniform outside diameter and insidediameter which is not available in the prior art.

In the present invention, two 10" inch inside diameter profile wallpipes were joined using a coupler having a width of approximately 3"inches. This resulted in a sufficient amount of mating surface to createa strong, uniform and water tight joint. The 3" inch coupler required achannel with a depth of at least 1-1/2" inches in each pipe forreceiving the coupler. As the diameter of pipes to be joined increases,the size and width of coupler also increases. For example, to join two42" inch inside diameter pipes would required a coupler withapproximately an 8" inch width.

The thickness of the coupler is determined by the "profile" distance(the distance between the inner wall and outer wall) of the pipes to bejoined. For example one manufacturer produces a polyethylene pipe with10" inch inside diameter and an outside diameter of approximately 11.20inches. The inner and outer wall thicknesses of this particular profilewall pipe range between 0.065" inches to 0.079" inches. Therefore, thisparticular pipe has a profile distance just over 1 inch, calculated bysubtracting the sum of the inside diameter and the inner and outer wallthicknesses from the outside diameter. A 36" inch inside diameterprofile wall pipe from the same manufacturer has a wall thickness ofapproximately 0.195 to 0.228 inches and an outside diameter ofapproximately 40.65 inches resulting in a profile distance between theinner and outer walls over 4 inches.

In the preferred embodiment, coupler 22 includes an electricalresistance element 32, shown in FIG. 5, to provide the heat required tofuse coupler 22 to inner wall 14 and outer wall 12 of pipes 10. Theelectrical resistance element of the preferred embodiment, shown in FIG.6 before attachment to coupler 22, is a stainless steel screen 33.Screen 33 has ends 34 and 36 with terminal pins 38 and 40 electricallyconnected thereto. Screen 33 also has a length, measured from end 34 toend 36 and width, measured from side 35 to side 37, sufficient to coverthe exterior surface 24 of coupler 22.

Ends 34 and 36 are configured to allow overlap of end 34 with end 36around the circumference without making contact therebetween. Thisprevents shorting the electrical resistance element 32 when it isenergized and allows uniform heating over entire exterior surface 24 ofcoupler 22. Terminal pins 38 and 40 are located at opposite ends ofscreen 33 to provide for electrical connection of a power source (notshown) to screen 33 for energization of screen 33.

Terminal pins 38 and 40 are electrically connected to electricalresistance element 32. In the preferred embodiment, terminal pins 38 and40, shown in FIG. 7 before attachment to screen 33, have a base 44 andan extension 46. Base 44 has a larger circumference than extension 46 toallow extension to be inserted in an opening of screen 33 withoutpulling base 44 through opening 48. Terminal pins 38 and 40 may be madeof any type of electrically conductive material such as copper orstainless steel but must be of sufficient gauge to allow transfer ofenough electrical current to the electrical resistance element 32 asrequired to fuse the coupler 22 to channel 20.

Referring to FIG. 4, terminal pins 38 and 40 are attached to ends 34 and36 and positioned to be aligned with the center of coupler 22,equidistant from sides 35 and 37, when screen 33 is wrapped aroundcoupler 22. Terminal pins 38 and 40 are positioned in this fashion toprotrude between abutted end walls 18 of pipes 10 to allow electricalconnection of screen 33 in channel 20 to an external power source (notshown).

To energize screen 33, the power source (not shown) is connected acrossto terminal pins 38 and 40 of screen 33 by connecting the positive leadfrom the power source to one terminal, for example 38, and the negativelead from the power source to the second terminal, for example 40. Aselectrical current is run through screen 33, screen dissipates heat tocoupler 22 and pipes 10.

Referring to FIG. 5, after coupler 22 is inserted into channel 20,terminal pins 38 and 40 are connected to a power source (not shown), asdescribed above, which supplies a predetermined amount of electricalcurrent sufficient to heat electrical resistance element 32 and softencoupler 22 and the appropriate mating surface of the pipes. Theinterference fit between coupler 22 and pipe 10 develops sufficientbonding pressure as coupler 22 softens and expands causing pressure atthe inner wall 14 and outer wall 12. Inner wall 14 and outer wall 12also are heated from energization of electrical resistance element 32and this produces a uniform and strong fusion bond between coupler 22and pipes 10.

In the preferred embodiment, electrical resistance element 32 isembedded into the exterior surface 24 of coupler 22. The electricalresistance element can also be attached to the interior surface 26 ofcoupler 22. In the alternative, electrical resistance element 32 can beembedded in inner wall 14 or outer wall 12 of the pipes. The exactlocation of electrical resistance element 32 is not critical as long asit is within sufficient proximity to both the coupler 22 and at leastone pipe wall 12 or 14 so that upon energization of electricalresistance element 32, coupler 22 will be fused to pipes 10.

Electrical resistance element 32 of the preferred embodiment iscomprised of stainless steel screen 33. However, any electricallyconductive material, such as wire, screen, mesh, or helical resistancewire, is acceptable provided upon energization it produces sufficientheat to fuse coupler 22 within channel 20 of each pipe. The use ofscreen 33 decreases the likelihood of an electrical short as oftenoccurs with an electrical resistance element made of a single wire.

For example, as shown in FIG. 8, electrical resistance element is formedby wrapping a single wire 42 in successive turns around coupler 22toward each edge 28 and 30. During insertion of coupler 22 into channel20, due to the required interference fit, one turn of single wire 42could be forced into an adjacent turn of wire thereby shorting thecircuit and preventing electricity from conducting past the short. Thiswould result in only a portion of coupler 22 being fused to the pipes.

In the preferred embodiment, electrical resistance element 32 isembedded into the exterior surface 24 of coupler 22. It is preferredthat electrical resistance element 32 be sufficiently held on coupler 22to prevent the electrical resistance element 32 from moving during theinsertion of the coupler 22 into the channel 20 due to the interferencefit between coupler 22 and channel 20. If a stronger joint is desired, asecond electrical resistance element can be added to the interiorsurface 26 of coupler 22. Proper placement of electrical resistanceelement 32 in channel 20 provides for a continuous and uniform fusionweld at the joint.

Coupler 22 can be manufactured with electrical resistance element 32embedded therein or the embedding can be accomplished in the field. Inthe manufacturing process, electrical resistance element 32 can beembedded using techniques known in the art currently used formanufacturing electrofusion collars or inserts. In the alternative,electrical resistance element 32 can be placed around the coupler 22 andcovered with a like thermoplastic material. If electrical resistanceelement 32 is to be attached to inner wall 14 or outer wall 12 of pipe10, the same techniques can be used when manufacturing the pipes toattach electrical resistance element in channel 20.

In the field, electrical resistance element 32 can be embedded intoexterior surface 24 of coupler 22 by wrapping electrical resistanceelement 32 around coupler 22 and applying pressure to urge it into theexterior surface of coupler 22 while energizing electrical resistanceelement 32. The pressure can be applied using a clamp (not shown), or abinder (not shown) which can be shaped around coupler 22 and electricalresistance element 32.

FIG. 5 is a cross sectional view showing coupler 22 inserted in channel20 joining the pipes 10 prior to energization of electrical resistanceelement 32. As is shown, end walls 18 of pipes 10 are abutting againstone another and the exterior surface 24 of coupler 22 is adjacent theouter walls 12 of pipes 10. The interior surface 26 of the coupler 22 isadjacent the inner walls 14 of pipes 10. Electrofusion element 32 ispositioned between exterior surface 24 and outer walls 12 of pipes 10.Terminal pins 38 and 40 extend between abutted end walls 18 to allowconnection to power source (not shown).

Any power source capable of supplying adequate power to the electricalresistance element is acceptable for the present invention. For example,a variable current electrical welder is acceptable as are any type ofpower supplies used in the prior art for fusion welding of thermoplasticpipes using electrofusion collars and inserts. Preferably, anelectrofusion control unit is used for the power source, as are known inthe art, to furnish a predetermined amount of electrical current whichis required to heat the screen 33 and partially melt exterior surface 24of coupler 22 and inner wall 14 and outer wall 12 of pipes 10.

The coupler 22 expands during heating at a greater rate than the pipewalls due to the attachment of the electrical resistance elementthereon. This develops additional internal bonding pressure to produce afusion bond of adequate depth and continuity to form a pressure tightpipe joint, that is substantially flush with the pipe walls, leaving nointerior flow restrictions or exterior collars or obstructions.

A variety of power sources exist in the art which will supply thecorrect amount of current for each pipe size. Existing electrofusioncontrol units are capable of measuring heat levels and/or maintaining ameasured welding time at a predetermined current to form a structurallysound leak-proof joint upon cooling. However any experienced fusionwelder with charts showing the recommended current and heating time foreach size and composition of pipe, is capable of forming a leak-proofjoint using any available, variable ampere, direct current welder, as apower source.

FIG. 9 shows a complete joint using the present invention after screen33 has been energized and the fusion weld is complete. As is shown, thepresent invention results in a substantially flush joint both interiorand exterior and a substantially uniform outside diameter and insidediameter throughout the length of the joined pipes.

FIGS. 10 through 12, show the first embodiment of the present inventionas described above. The only difference between these embodiments beingthe shape of the profile space 11 in the profile wall pipe used. Thedetailed description above is equally applicable to round profile wallpipe as discussed above with like numbers referring to like elements.

Having described the apparatus, a method for forming the pipe joint ofthe first embodiment between two pipes is explained. As noted above, thefirst embodiment is particularly adapted for use with profile wall pipe,and produces a pipe joint with substantially flush interior and exteriorsurfaces. This method is equally applicable to solid wall thermoplasticpipe having a sufficient wall thickness to allow the formation of achannel 20 between an inner wall 14 and an outer wall 12. The methodwill be explained assuming profile wall pipe, however, the onlyrequirement for the present invention is that the pipe be capable ofhaving a channel 20 at an end wall 18 for receiving a coupler 22therein.

To practice the method of the present invention, pipes 10 are modifiedto include a channel 20 in each end wall 18. This can be performed byrouting or machining taking care not to remove inner wall 14 or outerwall 12 in the process. In profile wall pipe, the creation of channel 20is made simpler by the lack of a solid wall at end walls 18. As statedabove, profile wall pipe is manufactured having a profile space 11formed by inner wall 14 outer wall 12 and rib 16. Therefore, at the endwall 18 of a section of profile wall pipe 10, profile space alreadyexists prior to routing. However, removal of rib 16 is required in orderto form channel 20 with a minimum depth the entire circumference of endwall 18.

The amount of rib 16 which must be removed is dependent upon the depthof channel 20 required, the size of rib 16 and the width of profilespace 11. It is preferable to have the depth of channel 20 uniform,however, all that is required is that the shallowest segment of channel20 be greater than or equal to 1/2 the width of coupler 22. Afterchannel 20 is formed in end walls 18 of pipes 10, the coupler can beinserted into the channel.

In the preferred embodiment, edges 28 and 30 of the coupler are beveledto easier insertion of coupler 22 into channel. In the preferredembodiment, electrical resistance element 32 is embedded into exteriorsurface 24 of coupler 22. If electrical resistance element 32 is notattached to coupler 22, this must be accomplished prior to insertion ofcoupler into pipe 20. An electrical resistance element, such as screen33 is wrapped around exterior surface 24 of coupler 22. A clamp (notshown) or adjustable binder (not shown) is tightened over screen 33urging screen 33 onto coupler 22. Terminal pins 38 and 40 should bealigned in the center of coupler 22 equidistant from edges 28 and 30.This allows the extension of terminal pins 38 and 40 to extend betweenabutment of end walls 18 after coupler 22 is inserted into channel 20 ofeach pipe 10 to be joined.

To embed screen 33 into exterior surface 24 of coupler 22, power source(not shown) is electrically connected to terminal pins 38 and 40. Apositive lead is attached to one terminal, for example 38 and thenegative lead from power source is attached to other terminal pin, forexample 40. Screen 33 is then energized by power source (not shown)causing electrical current to flow through screen 33. Screen 33 shouldbe energized long enough to attach screen 33 sufficiently to coupler 22to hold screen 33 in place during insertion of coupler 22 into channel20. In the preferred embodiment, screen 33 is energized until coupler 22begins to protrude through openings in screen 33.

After attachment of screen 33 to coupler, coupler is inserted intochannel 20 of each channel 20. In the preferred embodiment, coupler 22is inserted to a depth of one-half of its width into each pipe 10. Thisallows abutment of end walls 18 of each pipe 10 and provides an matingsurface between coupler 22 and each pipe 10. After coupler 22 isinserted into channel 20 of each pipe 10, screen 33 is energized, asdescribed above by connecting power source to terminal pins 38 and 40.Screen should be energized for a sufficient duration to cause pipes 10to be fusion welded to coupler 22. The amount of power and duration aredetermined by the types of thermoplastic material from which pipes 10are made and the amount of heat necessary to create the fusion weld. Asstated above, the amount of heat and power necessary is known in the artof electrofusion welding using electrofusion couplers and inserts.

A second embodiment of the present invention for use on solid wallthermoplastic pipes is shown in FIGS. 13-17. A first solid wall pipe 50and a second solid wall pipe 52 can be joined using the principles ofthe present invention by modifying a first pipe end 54 and a second pipeend 56 for use with electrical resistance element 32. This embodimentcreates a substantially flush interior and exterior pipe joint and asubstantially uniform outside diameter and inside diameter. As shown,first pipe 50 and second pipe 52 have substantially equal outsidediameters, substantially equal inside diameters and each has asubstantially uniform circumference. First end 54 and second end 56 havesubstantially the same inside diameter and outside diameter as pipes 50and 52.

The outside diameter of first end 54 is machined or shaved to form amale nipple 58. In order to allow for a pipe joint having asubstantially flush interior and substantially flush exterior, nipple 58is formed by decreasing outside diameter of first end 54 and therebydecreasing the wall thickness. At one end of nipple 58 is shoulder 61where the nipple 58 meets the remainder of first pipe 50 and is formedby the difference in outside diameter between nipple 58 and first pipe50. At the other end of nipple is nipple edge 60 which can be beveled orchamfered. Between nipple edge 60 and shoulder 61 is nipple exteriorsurface 74.

The inside diameter of second end 56 is machined or shaved to form afemale socket 62. In order to allow for a pipe joint having asubstantially flush interior and substantially flush exterior, socket 62is formed by increasing inside diameter of second end 56 and therebydecreasing the wall thickness. Referring to FIG. 14, one end of socket62 is end wall 63 of second pipe 52 and at the other end of socket islip 65 formed by the difference in inside diameter between second pipe52 and socket 62. Between lip 65 and end wall 63 is socket interiorsurface 76.

The amount of decrease in wall thickness (and increase in insidediameter) of second end 56 is approximately equal to the decrease inwall thickness (and decrease in outside diameter) of first end 54. Inthe preferred embodiment, the wall thickness of both first end 54 andsecond end 56 are decreased by approximately one-half of the originalwall thickness, however, any amount of decreased in wall thickness isacceptable provided the amount of decrease is equal in both first end 54and second end 56 to provide a flush interior and exterior surface.

Referring to FIG. 15, the length of nipple 58, measured from shoulder 61to the nipple edge 60, is equal to the depth of socket 62, measured fromend wall 63 to lip 65. This allows the nipple 58 to have an interferencefit within the socket 62 with the exterior surface 74 of the nipple 58adjacent to interior surface 76 of socket 62, shoulder 61 of first pipe50 abutted against end wall 63 of second pipe 52 and lip 65 of secondpipe 52 abutted against nipple edge 60 of first pipe 50. The resultingjoint having substantially flush interior and exterior and substantiallyuniform outside and inside diameters.

The nipple 58 has an electrical resistance element 64 embedded into theexterior surface 74 of the nipple. The electrical resistance element 64is substantially identical to the electrical resistance element 32 ofthe first embodiment discussed above except for the configuration of theends 66 and 68 and location of the terminal pins 70 and 72. However, thecomposition and characteristics of the electrical resistance element 64and the terminal pins 70 and 72 are identical to the electricalresistance element 32 and the terminal pins 38 and 40, respectively.Therefore, the discussion above relate thereto is equally applicableherein.

In this second embodiment, as shown in FIG. 13, the electricalresistance element 64 has a length, measured between the ends 66 and 68,sufficient to substantially cover the exterior surface 74 of the nipple58 and a width substantially equal to the length of nipple 58. Ends 66and 68 are cut at the right angle to the length of the electricalresistance element and do not overlap. This allows extension of theterminal pins 70 and 72 between shoulder 61 of the first pipe 50 and theend wall 63 of the second pipe 52 for connection to power source (notshown).

The attachment of electrical resistance element 64 in this secondembodiment to the exterior surface 74 of the nipple 58 is identical tothe attachment of electrical resistance element 32 to exterior surface24 of coupler 22 described above. In the alternative, electricalresistance element 64 may be attached to interior surface 76 of socket62. As set forth in detail above, electrical resistance element 64should be sufficiently close to both the interior surface 74 and theexterior surface 76 to cause fusion of the socket 62 within nipple 58 onenergization of electrical resistance element 64.

FIG. 16 shows the nipple 58 inserted within socket 62. As is shown,exterior surface 74 of the nipple 58 is adjacent the interior surface 76of socket 62. The nipple edge 60 is abutted against the lip 65 and theend wall 63 is abutted against shoulder 61. The electrical resistanceelement 64 is embedded into the exterior surface 74 of the nipple 58 andis sufficiently close to the interior surface 76 of socket 62 to causefusion of the nipple 58 to the socket 62 upon energization of theelectrical resistance element 64. Terminal pins 70 and 72 extend betweenthe abutted end wall 63 and the shoulder 61 to allow connection to powersource (not shown).

Any power source capable of supplying adequate power to the electricalresistance element 64 is acceptable for the present invention. Forexample, a variable current electrical welder is acceptable as are anytype of power supplies used in the prior art for fusion welding ofthermoplastic pipes using electrofusion collars and inserts.

Preferably, an electrofusion control unit is used for the power source,as are known in the art, to furnish a predetermined amount of electricalcurrent which is required to heat the screen 78 and partially meltexterior surface 74 of the nipple 58 and the interior surface 76 ofsocket 62. The nipple 58 expands during heating at a greater rate thanthe interior surface 76 of the socket 62 due to the attachment of theelectrical resistance element thereon. This develops additional internalbonding pressure to produce a fusion bond of adequate depth andcontinuity to form a pressure tight pipe joint, that is substantiallyflush with the pipe walls, leaving no interior flow restrictions orexterior collars or obstructions.

A variety of power sources exist in the art which will supply thecorrect amount of current for each pipe size. Existing electrofusioncontrol units are capable of measuring heat levels and/or maintaining ameasured welding time at a predetermined current to form a structurallysound leak-proof joint upon cooling. However any experienced fusionwelder with charts showing the recommended current and heating time foreach size and composition of pipe, is capable of forming a leak-proofjoint using any available, variable ampere, direct current welder, as apower source.

FIG. 17 shows a complete joint using the present invention after screen78 has been energized and the fusion weld is complete. As is shown, thepresent invention results in a substantially flush joint both interiorand exterior and a substantially uniform outside diameter and insidediameter throughout the length of the joined pipes.

Having described the apparatus, a method for forming the pipe joint of asecond embodiment between two solid thermoplastic pipes is explained. Topractice the method of the present invention, a first pipe 50 ismodified at a first end 54 to form a nipple 58 and a second pipe 52 ismodified to form a socket 62 for receiving nipple 58 therein.

First pipe 50 and second pipe 52 have substantially equal outsidediameters, substantially equal inside diameters and each has asubstantially uniform circumference. First end 54 and second end 56 havesubstantially the same inside diameter and outside diameter as pipes 50and 52.

To form nipple 58 the outside diameter of first end 54 is machined orshaved by decreasing outside diameter of first end 54 and therebydecreasing the wall thickness. The inside diameter of second end 56 ismachined or shaved to form a female socket 62 by increasing insidediameter of second end 56 and thereby decreasing the wall thickness.

The amount of decrease in wall thickness of second end 56 isapproximately equal to the decrease in wall thickness of first end 54.The length of nipple 58, measured from shoulder 61 to nipple edge 60, isequal to the depth of socket 62, measured from end wall 63 to lip 65.This allows nipple 58 to have an interference fit within socket 62 withexterior surface 74 of nipple 58 adjacent to interior surface 76 ofsocket 62 and shoulder 61 of first pipe 50 to abut against lip 65 ofsecond pipe 52. The resulting joint having substantially flush interiorand exterior and substantially uniform outside and inside diameters.

After forming nipple 58 and socket 62, an electrical resistance element64 is embedded into the exterior surface 74 of nipple 58. An electricalresistance element, such as screen 78 is wrapped around exterior surface74 of nipple 58. A clamp (not shown) or adjustable binder (not shown) istightened over screen 78 urging screen 78 onto nipple 58. Terminal pins70 and 72 should be aligned adjacent shoulder 61. This allows theextension of terminal pins 70 and 72 to extend between abutment of endwall 63 and shoulder 61 after nipple 58 is fully inserted into socket62.

To embed screen 78 into exterior surface 74 of nipple 78, power source(not shown) is electrically connected to terminal pins 70 and 72. Apositive lead is attached to one terminal pin, for example 70 and thenegative lead from power source is attached to other terminal pin, forexample 72. Screen 78 is then energized by power source (not shown)causing electrical current to flow through screen 78. Screen should beenergized long enough to attach screen 78 sufficiently to nipple 58 tohold screen 78 in place during insertion of nipple 58 into socket 62. Inthe preferred embodiment, screen 78 is energized until nipple 58 beginsto protrude through openings in screen 78.

After attachment of screen 78 to nipple 58, nipple 58 is inserted intosocket 62. In the preferred embodiment, nipple 58 is fully inserted intosocket 62. This allows nipple edge 60 to abut lip 65 and shoulder 61 toabut end wall 63 and provides an mating surface between exterior surface74 of nipple 58 and interior surface 76 of socket 62. After nipple 58 offirst pipe 50 is inserted into socket 62 of second pipe 52, screen 78 isenergized, as described above by connecting power source to terminalpins 70 and 72. Screen 78 should be energized for a sufficient durationto cause nipple 58 to be fusion welded within socket 62. The amount ofpower and duration are determined by the types of thermoplastic materialfrom which pipes are made and the amount of heat necessary to create thefusion weld. The amount of heat and power necessary is known in the artof electrofusion welding using electrofusion couplers and inserts.

Any power source capable of supplying adequate power to electricalresistance element 64 is acceptable for the present invention. Forexample, a variable current electrical welder is acceptable as are anytype of power supplies used in the prior art for fusion welding ofthermoplastic pipes using electrofusion collars and inserts.

After inserting nipple 58 with electrical resistance element 64 attachedthereto into socket 62, a predetermined amount of electrical current isapplied to the terminal pins 70 and 72 and the thermoplastic material onthe nipple 58 softens and expands as it melts into the socket 62providing additional fusion pressure as the socket interior surface 76softens to fuse the two pipe sections together with adequate depth anduniformity to make a leak free and strong pipe joint with no interiorflow obstructions or exterior collar or protrusions. The joint is madewithout the use of butt fusion machines or line up equipment necessaryto provide longitudinal pressure to join the two pipe sections asrequired in the prior art.

There are various changes and modifications which may be made to theinvention as would be apparent to those skilled in the art. However,these changes or modifications are included in the teaching of thedisclosure, and it is intended that the invention be limited only by thescope of the claims appended hereto.

What is claimed is:
 1. An electrofusion weld assembly for uniting a pairof substantially identical profile wall thermoplastic pipes, each of thepipes having an outer wall, an inner wall, an end wall, a helical riblocated generally between the inner wall and the outer wall and joiningthe inner wall to the outer wall, and a recessed channel in the end wallfor receiving a coupler therein, said weld assembly comprising:a coupleruniting the end walls of the profile wall pipes and positioned withinthe channels, the coupler having an exterior surface adjacent the outerwalls of the profile wall pipes and an interior surface adjacent theinner walls of the profile wall pipes; and an electrical resistanceelement sufficiently close to both the coupler and the profile wall pipewalls to cause the profile wall pipes to fuse to the coupler onenergization of the electrical resistance element thereby forming a pipejoint having a substantially constant inner diameter and a substantiallyconstant outer diameter.
 2. The electrofusion weld assembly of claim 1wherein the electrical resistance element is within the channel.
 3. Theelectrofusion weld assembly of claim 2 wherein the electrical resistanceelement is embedded on the exterior surface of the coupler to cause theexterior surface of the coupler to fuse to the outer walls of theprofile wall pipes on energization of the coupler.
 4. The electrofusionweld assembly of claim 1 wherein at least one edge of the coupler isbeveled.
 5. The electrofusion weld assembly of claim 1 wherein theelectrical resistance element is comprised of stainless steel screen. 6.The electrofusion weld assembly of claim 5 further comprising a secondelectrical resistance element embedded on the interior surface of thecoupler to cause the interior surface of the coupler to fuse to theinner walls of the profile wall pipes on energization of the electricalresistance element.
 7. A method of coupling a pair of profile wallthermoplastic pipes, each of said pipes having an inner wall, an outerwall and a helical rib between the inner wall and outer wall and joiningsaid walls together, the method comprising the steps of:forming achannel between the inner wall and the outer wall at an end of each ofthe profile wall pipes by removing a section of the helical rib to apredetermined distance from the pipe ends; and positioning a couplerwithin the channel in each of the profile wall pipes thereby uniting theends of the pipes; and heating the coupler until the coupler is fusionwelded to each of the profile wall pipes thereby forming a pipe jointhaving a substantially constant inner diameter and a substantiallyconstant outer diameter.
 8. The method of claim 7 wherein the step ofheating the coupler is performed by electrofusion welding.
 9. The methodof claim 8 wherein the step of electrofusion welding comprises the stepsof:affixing an electrical resistance element to the coupler prior topositioning the coupler within the channels; and energizing theelectrical resistance element after the coupler is positioned within thechannels.
 10. The method of claim 9 wherein the step of affixing theelectrical resistance element to the coupler comprises the stepsof:positioning the electrical resistance element in contact with asurface on the coupler; and energizing the electrical resistance elementwhile applying pressure to the electrical resistance element to urge theelectrical resistance element into the surface of the coupler.
 11. Themethod of claim 10 wherein the step of forming the channel between theinner wall and the outer wall comprises the step of removing a segmentof the helical rib approximately one-half the width of the coupling. 12.The method of claim 11 wherein the step of removing the helical rib isperformed by routing.
 13. A profile wall thermoplastic piping systemcomprising:a plurality of profile wall pipes adapted to be joinedtogether end-to-end, each pipe having a generally cylindrical outer wallhaving a predetermined diameter, a generally cylindrical inner wallhaving a predetermined diameter and being substantially concentric withthe outer wall and a helical rib located generally between the innerwall and the outer wall and joining the inner wall and outer walltogether; an annular insert having an insert inner diameter slightlylarger than the outer wall diameter and an insert outer diameterslightly smaller than the outer wall diameter, the annular insert beingengageable with the pipes by insertion between the inner walls and theouter walls; and an electrical resistance element sufficiently close toboth the annular insert and the pipe walls to cause the pipes to fuse tothe coupler on energization of the electrical resistance element,thereby forming a pipe joint having a substantially constant innerdiameter and a substantially constant outer diameter.
 14. The profilewall piping system of claim 13 wherein the electrical resistance elementis comprised of stainless steel screen.
 15. The profile wall pipingsystem of claim 13 wherein the electrical resistance element is embeddedon an exterior surface of the annular insert.
 16. The profile wallpiping system of claim 13 wherein the electrical resistance element isembedded on an interior surface of the annular insert.
 17. The profilewall piping system of claim 13 wherein the annular insert has bevelededges.